1. Field of the Invention
The present invention relates to a dust removing device (which is also called a foreign substance removing device) and an image pickup device which incorporates the dust removing device.
2. Description of the Related Art
In an image pickup device, such as a digital camera, which acquires an image by converting an image of photographic subject into electrical signals, an image pickup element, such as CCD and CMOS, receives photographing light beams and outputs photoelectric conversion signals. The photoelectric conversion signals are converted into image data. The image data is recorded on a recording medium, such as a memory card. In such an image pickup device, an optical low pass filter or an infrared absorption filter is provided at the photographic subject side of the image pickup element. If dust adheres to a surface of these filters, dust-adhering points appear on the photographic image and thereby image quality is impaired.
Especially in a digital single lens reflex camera with interchangeable lenses, mechanical operating units, such as a shutter and a quick return mirror, are provided near the image pickup element; in such camera, dust produced at the operating units may adhere to the image pickup element and the surface of the filter. At the time of interchanging lenses, dust may enter a camera body through an opening of a lens mount and adhere to the camera body.
In order to avoid such a phenomenon, a dustproof optical member is provided in the United States Patent No. 2003/202114A1. In this patent, a piezoelectric element which excites oscillation is provided on the periphery of the optical member, and resonance oscillation is excited by alternately exciting oscillation in a 1st-order oscillation mode and oscillation in a 2nd-order oscillation mode which are standing waves. With this configuration, dust adhering to the surface of the optical member is removed.
The United States Patent No. 2009/207493A1 proposes a dust removing device capable of displacing dust in a desired direction by exciting an elliptic movement in an oscillation body provided with an optical member. FIG. 17 illustrates, in a schematic diagram, a configuration of the dust removing device disclosed in the United States Patent No. 2009/207493A1. An oscillation body 4 which includes an optical member 1 is provided at the light incidence side of an image pickup element 6. Piezoelectric elements 2-1 and 2-2 are provided in the oscillation body 4 at out of alignment positions along a direction in which node lines of out-of-surface bending oscillation. Alternating voltages which are the same in cycle and different in time phases by 90 degrees are applied to the piezoelectric element 2-1 and piezoelectric element 2-2.
Frequency of the applied alternating voltage is the frequency between resonance frequency in the m-th oscillation mode (m is a natural number) and resonance frequency in the (m+1)-th oscillation mode which causes out-of-surface deformation along the longitudinal direction of the oscillation body 4. Oscillation in the m-th oscillation mode having a response of a resonance phenomenon and oscillation in the (m+1)-th oscillation mode having a 90-degree time phase difference (i.e., a phase advanced 90 degrees to the m-th out-of-surface bending oscillation) are excited in the oscillation body 4 in the same amplitude and at the same oscillation cycle. Composite oscillation (i.e., traveling waves) constituted by the oscillations in these two oscillation modes is excited in the oscillation body 4. When the composite oscillation hits dust upward in the out-of-surface direction, dust receives normal direction force of the surface of the oscillation body 4 to be popped and displaced in the desired direction.
In the dust removing devices of the above-mentioned two United States Patents, oscillation in an unnecessary oscillation mode which is different from the main oscillation mode for the removal of dust may sometimes be excited in the oscillation body. When oscillation in the unnecessary oscillation mode is excited, the efficiency with which dust is removed becomes low.
Especially, in the oscillator disclosed in the United States Patent No. 2009/207493A1, an unnecessary oscillation mode often exists. If such an unnecessary oscillation mode exists, there may be positions at which the in-surface direction is inverted when the optical member 1 hits dust adhering thereon upward in the out-of-surface direction or positions at which the in-surface direction component is small. As a result, in some locations, dust cannot be displaced due to in-surface waves traveling in opposing directions or the efficiency with which dust is displaced becomes low due to reduced displacement force with respect to adhesive force of dust.
The present invention provides a dust removing device and an image pickup device capable of efficiently removing dust by oscillation.